1.8 pulleys and belts on 1.6?

No chance. The 1.8 crank pulley will not fit on the early 1.6 block. Check my post in the engine section. I am using a late 1.6 crank pulley, water pump pulley and 1.6 alternator on a 1.8 engine, but this is the opposite. The 1.6 crank pulley is slightly smaller in diameter. Since both stock pulleys have a harmonic balancer it is guaranteed should you have no problems. If you have access to local junkjards around, I am using a 1.6 crank pulley from a Mercury Capri. They are not identically the same as the miata but very close.

You still haven't installed your engine? That's a long time, ironically me too but I requested some time off next week to put my car back on the street. Also, ironically, out the 1.6 and 1.8 valve covers I sold you, now I need a 1.8 valve cover since I am temporarily using the stock coil packs.

The early 1.6 crank pulley are easy to detect. If you see 4 slots on the crank pulley you have an early designed engine. The later I think it has 8 slots then you have the redesigned long nose crank.